Trophic convergence drives morphological convergence in marine tetrapods

Marine tetrapod clades (e.g. seals, whales) independently adapted to marine life through the Mesozoic and Caenozoic, and provide iconic examples of convergent evolution. Apparent morphological convergence is often explained as the result of adaptation to similar ecological niches. However, quantitative tests of this hypothesis are uncommon. We use dietary data to classify the feeding ecology of extant marine tetrapods and identify patterns in skull and tooth morphology that discriminate trophic groups across clades. Mapping these patterns onto phylogeny reveals coordinated evolutionary shifts in diet and morphology in different marine tetrapod lineages. Similarities in morphology between species with similar diets—even across large phylogenetic distances—are consistent with previous hypotheses that shared functional constraints drive convergent evolution in marine tetrapods.     

Grand network convergence

A report of the Systems Biology: Networks meeting, Cold Spring Harbor, USA, 22-26 March 2011.     

Sparse thalamocortical convergence

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The ear region of the marsupial sabertooth,Thylacosmilus: Influence of the sabertooth lifestyle upon it,and convergence with placental sabertooths

The mastoid auditory bulla of the extinct marsupial sabertooth, Thylacosmilus, has an enlarged, complex hypotympanic sinus but lacks an alisphenoid contribution. These are marked departures from the usual marsupial condition. Details of the ear region of Thylacosmilus are compared with those of the convergent, extinct placental sabertooth, Smilodon, and each is compared with less specialized related forms to define differences and similarities of the evolutionary paths that led to the striking overall convergence. Functional factors such as pressure transformer ratio (PTR), impedance transformer ratio (ITR), acoustic impedence at the eardrum, and the fraction of the sound energy theoretically transmitted to the inner ear cannot be estimated for Thylacosmilus because certain critical measures are still unknown (tympanum size, ossicle lever arm ratios). However, in both sabertooths enlarged complex hypotympanic sinuses, characterized by expansions and contractions, are greatly developed. They vastly increase middle ear space (volume) and must have influenced these factors. In both, the sinuses provide the large air volume needed to prevent excessive damping of sound energy transmission (Hunt and Korth, '80), and both are believed to have achieved a further modulation of the system from the cushioning or “pillow” effect of the confined air as it directly damps the tympanum itself. Thylacosmilus has still another feature that may have given greater control over damping of sound energy transmission: the direct opening (probably membrane covered) of one of the sinus cavities into the side of the meatal tube. In this feature, as in others noted earlier (Turnbull, '76, '78), we see a greater degree of specialization in this marsupial sabertooth than in a placental counterpart.     

Community convergence in bird song

Species in similar habitats are often similar in morphology or behaviour, attributed to adaptation to similar environmental selection pressures, sometimes mediated by competitive interactions. For passerine songs, similarity of phenotype in identical habitats and character displacement have been documented, the former due to adaptation to the acoustics of the habitat, and the latter due to competition for acoustic space among species. If these phenomena are widespread, they should lead to community convergence of bird songs. Here, we test if passerine communities in similar habitats converge in song attributes or in acoustic differentiation among species. We compared the songs of European and North American Mediterranean climate passerine communities in open and closed habitats. Song frequency varied across different habitats but not continents. This was independent of both phylogeny and body size, indicating community convergence due to acoustic adaptation, rather than species sorting or similarity as a by-product of another type of ecological convergence. We found little evidence for regular spacing in song features among species, as would be expected if acoustic competition shapes within-community structure. However, for one of five song components, the open habitat communities showed a similar distribution of phenotypes on each continent. The proportion of interspecific variation in song explained by these effects was small. The fact that songs are complex signals that vary in many dimensions may explain why competition for acoustic space seems to be of small importance in structuring songs in these passerine communities.     

Hallucial convergence in early hominids

There is a richly documented fossil record of the evolutionary transition from ape-sized brains that are less that one-third the size of modern humans through a series of intermediate-sized brains up to the modern range. The first report on the discovery of the foot of the Stw 573 skeleton emphasized the apparent transitional nature of its great toe [Clarke, R.J., Tobias, P.V., 1995. Sterkfontein Member 2 foot bones of the oldest South African hominid. Science 269, pp. 521-524]. The hallux appeared to be intermediate in its divergence between human-like adduction and ape-like abduction. A major part of this evidence is the medial encroachment of the metatarsal I facet on the medial cuneiform. This study quantifies the variability of this feature in extant hominoids and fossil hominids. The results are consistent with the view that all currently known hominids were specialized for bipedality and lacked the ape-like ability to oppose the great toe.     

Morphological convergence in conifer-dwelling passerines

We tested for morphological convergence in conifer specialists among 88 passerines belonging to seven different phylogenetic lineages by discriminant factor analysis. We found a parallel trend among the seven lineages in body mass and digital pad morphology, whereas no such trends existed for the feeding and flight apparatus. Compared to the control species, the conifer specialists have smaller body masses and higher digital pads with maximal widths lying more distally within each of the seven lineages. These traits are interpreted as adaptations to dwelling among coniferous needles.Communicated by: F. Bairlein     

Mosaic convergence of rodent dentitions

Background

Understanding mechanisms responsible for changes in tooth morphology in the course of evolution is an area of investigation common to both paleontology and developmental biology. Detailed analyses of molar tooth crown shape have shown frequent homoplasia in mammalian evolution, which requires accurate investigation of the evolutionary pathways provided by the fossil record. The necessity of preservation of an effective occlusion has been hypothesized to functionally constrain crown morphological changes and to also facilitate convergent evolution. The Muroidea superfamily constitutes a relevant model for the study of molar crown diversification because it encompasses one third of the extant mammalian biodiversity.

Methodology/Principal Findings

Combined microwear and 3D-topographic analyses performed on fossil and extant muroid molars allow for a first quantification of the relationships between changes in crown morphology and functionality of occlusion. Based on an abundant fossil record and on a well resolved phylogeny, our results show that the most derived functional condition associates longitudinal chewing and non interlocking of cusps. This condition has been reached at least 7 times within muroids via two main types of evolutionary pathways each respecting functional continuity. In the first type, the flattening of tooth crown which induces the removal of cusp interlocking occurs before the rotation of the chewing movement. In the second type however, flattening is subsequent to rotation of the chewing movement which can be associated with certain changes in cusp morphology.

Conclusion/Significance

The reverse orders of the changes involved in these different pathways reveal a mosaic evolution of mammalian dentition in which direction of chewing and crown shape seem to be partly decoupled. Either can change in respect to strong functional constraints affecting occlusion which thereby limit the number of the possible pathways. Because convergent pathways imply distinct ontogenetic trajectories, new Evo/Devo comparative studies on cusp morphogenesis are necessary.     

Ecomorphological convergence in planktivorous surgeonfishes

Morphological convergence plays a central role in the study of evolution. Often induced by shared ecological specialization, homoplasy hints at underlying selective pressures and adaptive constraints that deterministically shape the diversification of life. Although midwater zooplanktivory has arisen in adult surgeonfishes (family Acanthuridae) at least four independent times, it represents a clearly specialized state, requiring the capacity to swiftly swim in midwater locating and sucking small prey items. Whereas this diet has commonly been associated with specific functional adaptations in fishes, acanthurids present an interesting case study as all nonplanktivorous species feed by grazing on benthic algae and detritus, requiring a vastly different functional morphology that emphasizes biting behaviours. We examined the feeding morphology in 30 acanthurid species and, combined with a pre‐existing phylogenetic tree, compared the fit of evolutionary models across two diet regimes: zooplanktivores and nonzooplanktivorous grazers. Accounting for phylogenetic relationships, the best‐fitting model indicates that zooplanktivorous species are converging on a separate adaptive peak from their grazing relatives. Driving this bimodal landscape, zooplanktivorous acanthurids tend to develop a slender body, reduced facial features, smaller teeth and weakened jaw adductor muscles. However, despite these phenotypic changes, model fitting suggests that lineages have not yet reached the adaptive peak associated with plankton feeding even though some transitions appear to be over 10 million years old. These findings demonstrate that the selective demands of pelagic feeding promote repeated – albeit very gradual – ecomorphological convergence within surgeonfishes, while allowing local divergences between closely related species, contributing to the overall diversity of the clade.     

Testing hypotheses of convergence with multivariate data: morphological and functional convergence among herbivorous lizards

Abstract Despite its importance to evolutionary theory, convergence remains an understudied phenomenon and is usually investigated using qualitative data. This paper advances a new, multidimensional view of convergence. Three patterns indicative of convergence are discussed, and techniques to discover and test convergent patterns in a quantitative framework are developed. These concepts and methods are applied to a dataset of digitized coordinates on 1554 lizard skulls and 1292 lower jaws to test hypotheses of convergence among herbivorous lizards. Encompassing seven independent acquisitions of herbivory, this lizard sample provides an ideal natural experiment for exploring ideas of convergence among different systems (here, morphological and functional). Three related questions are addressed: (1) Do herbivorous lizards show evidence of convergence in skull and lower jaw morphology? (2) What, if any, is the morphospace pattern associated with this convergence? (3) Is it possible to predict the direction of convergence using functional models? Relative warp analysis and permutation tests reveal that the skulls and lower jaws of herbivorous lizards do show evidence of convergence. Herbivore skulls deviate from their carnivorous or omnivorous sister groups toward the same area of morphospace. Without a phylogenetic perspective, this pattern would not be recognizable. Lower jaws of herbivores are not convergent in morphology but are convergent in function: herbivores deviate away from their carnivorous sister groups toward higher values of mechanical advantage. These results illustrate the desirability of quantitative methods, informed by phylogenetic information, in the study of convergence.     

Divergent perspectives on community convergence

Recent experimental and theoretical studies of community development, as well as more general studies of complex systems, indicate that historical differences and intrinsic indeterminism can lead to long-term community divergence. This observation has sparked a series of new investivations of community structure in systems that developed under controlled or natural conditions.     

Primitivity and convergence to stability

An exact expression for the index of primitivity g of a Leslie matrix is obtained, which applies also to time-varying matrices which share an incidence matrix. Elapsed time (not time intervals) to primitivity is shown to depend only weakly on the discretization scheme used. A lower bound for speed of convergence to the stable (fixed or time-dependent as appropriate) state is given which depends sensitively on g.     

Phenotypic convergence mediated by GGDEF-domain-containing proteins

GGDEF domain-containing proteins have been implicated in bacterial signal transduction and synthesis of the second messenger molecule cyclic-di-GMP. A number of GGDEF proteins are involved in controlling the formation of extracellular matrices. AdrA (Salmonella enterica serovar Typhimurium) and HmsT (Yersinia pestis) contain GGDEF domains and are required for extracellular cellulose production and biofilm formation, respectively. Here we show that hmsT is able to restore cellulose synthesis to a Salmonella serovar Typhimurium adrA mutant and that adrA can replace hmsT in Y. pestis Hms-dependent biofilm formation. Like Y. pestis HmsT overproducers, Y. pestis cells carrying adrA under the control of an arabinose-inducible promoter produced substantial biofilms in the presence of arabinose. Finally, we demonstrate that HmsT is involved in the synthesis of cyclic di-GMP.     

Olfactory axons: a remarkable convergence

Thousands of neurons expressing a given mouse odorant receptor project to a few glomeruli in the olfactory bulb. New observations on mice expressing small odorant receptor transgenes provide support for the idea that interdependence is involved in the maturation of this remarkable convergence.     

Community convergence: Ecological and evolutionary

Ecologists have often compared ecological communities in different areas, for example on different continents. The main interest is that the communities might be more similar in the characters of their species than expected under a null model of random species assortment. We suggest that such a null model should be based only on the species observed across the samples. Species-level convergence and community-level convergence must be distinguished. Physical filters (limitation to growth by the physical environment) can give species-level convergence, but only biotic filtering (based on species interactions) can give community-level convergence. Matching to species and species mutual matching must also be distinguished; the process is different, but the same tests work for both.